FIG. 1 is a mechanical diagram showing a variable speed drive apparatus. The rotational torque is transmitted to a machine 5 from an electric motor 2 through a drive shaft 4 having a torsional rigidity of K [Kgm/rad].
A variable speed control unit 1 controls the speed of the electric motor 2 by using a signal, which is detected by a speed detector 3 attached to the electric motor 2, as a speed feedback signal.
FIG. 2 is a block diagram showing a speed control unit including the well-known torsional vibration system. As shown in FIG. 2, the speed control unit 11 has an integrator having a proportion gate A and a time constant .tau..sub.I and amplifies a deviation between a speed command N.sub.REF as indicated at 10 and a speed feedback signal N.sub.MFB to output a torque command signal T.sub.REF. When the torque command signal T.sub.REF is inputted to a motor torque controller 17, this motor torque controller 17 controls the torque of the electric motor with a linear delay time constant .tau..sub.T. Incidentally, the speed feedback signal N.sub.MFB is prepared from the rotational speed N.sub.M of the electric motor through a linear delay first-order lag element 16 (wherein .tau..sub.F : a speed detection filter delay time constant).
A motor torque T.sub.M is controlled in accordance with the aforementioned torque command signal T.sub.REF.
Reference numeral 12 designates a block indicating the mechanical time constant .tau..sub.M of the electric motor; numeral 13 designates a block indicating a torsional time constant .tau..sub.V ; and numeral 14 designates a block indicating the mechanical time constant .tau..sub.L of the load.
On the other hand, numeral 15 designates the load torque T.sub.L2 upon the machine 5, and letters N.sub.L designate the speed of the load.
In the block diagram of FIG. 2, a ramping (or linearly accelerated) speed command N.sub.REF is fed to the input. If a torsion is established in the drive shaft 4 when the motor speed N.sub.M and the load speed N.sub.L occur, a transient gain of the speed control system abruptly occurs with a mechanical resonance frequency of the rotational motion, which is determined by the torsional rigidity of the drive shaft, the inertia of the electric motor, and the combined inertia of the machine and the load. As a result, there occur periodic speed fluctuations which are detrimental to the machine facilities, as shown in FIG. 3.
As means for solving this problem, according to the prior art, a vibration suppressing filter 18 is inserted at the output side of the speed controller 11, as shown in FIG. 4, so as to reduce the transient gain at the resonance point of the mechanical system. The torsional vibration suppressing filter 18 is given a transmission function, as expressed by the following formula: EQU 1/{(s/.omega..sub.F).sup.2 +2.delta..sub.F (s/.omega..sub.F)+1}(1),
wherein: .omega..sub.F designates a transient gain reduction starting angular frequency; .delta..sub.F designates a filter characteristic constant; and s designates a Laplacian operator.
In the prior art, the filter angular frequency .omega..sub.F and the filter characteristic constant .delta..sub.F, as appearing in the above Formula, and the proportional gain A of the speed controller 11 are adjusted for all facilities to select a filter constant for reducing the influence of the torsional vibration.
In the prior art, however, the speed control system is made unstable if the filter constants .omega..sub.F and .delta..sub.F are merely selected for all facilities and adjusted, because the phase delay angle between the speed command and the speed feedback signal of the speed control system is further increased by incorporating the filter. This frequently makes it necessary to drastically reduce the proportional gain A of the speed controller so that the responsiveness of the speed control system is drastically lowered. Thus, there arises the problem that the speed control responding characteristics necessary for the facilities cannot be attained.
In addition, FIG. 5 is another mechanical diagram showing an ordinary variable speed drive apparatus. To the machine 5, the rotational torque is transmitted by the electric motor 2 through the drive shaft 4 having a low torsional rigidity K.sub.1. To the rotor 6 of the electric motor 2 at the side opposite the load, the rotational torque is transmitted by the electric motor 2 through a drive shaft 7 having a torsional rigidity K.sub.2. To the shaft of the rotor 6, the speed detector 3 is attached for detecting the speed of the rotor 6.
The variable speed control unit 1 is fed as a speed feedback signal N.sub.FB with the signal, which is prepared from the signal detected by the speed detector 3 through the filter having a first-order lag element, to control the speed of the electric motor 2.
FIG. 6 is a block diagram showing a speed control including the torsional vibration system shown in FIG. 5. As shown in FIG. 6, the speed controller 11 has an integrator having the proportional gain A and the time constant .tau..sub.I and amplifies the deviation between the signal command N.sub.REF, as designated at 10, and the speed feedback signal N.sub.FB to output the torque command signal T.sub.REF. When the torque command signal T.sub.REF is inputted to the motor torque controller 17, this motor torque controller 17 controls the torque of the electric motor with the linear delay time constant .tau..sub.T. Incidentally, the speed feedback signal N.sub.FB is prepared through a first-order lag element 19 (wherein .tau..sub.F0 designates a speed detection filter delay time constant) from the signal detected by the speed detector 3.
The motor torque T.sub.M is controlled in accordance with the aforementioned torque command signal T.sub.REF.
The numeral 12 designates the block indicating the mechanical time constant .tau..sub.M of the electric motor; the numeral 13 designates a block indicating the torsional time constant .tau..sub.V1 of the drive shaft of the electric motor at the load side; the numeral 14 designates a block indicating the mechanical time constant .tau..sub.L of the load; the numeral 15 designates the load torque T.sub.L2 to be applied to the machine 5; and the letters N.sub.L designate the speed of the machine. Moreover, numeral 20 designates a block indicating a torsional time constant .tau..sub.V2 of the drive shaft 7 of the electric motor at the side opposite the load, and numeral 21 designates a block indicating the mechanical time constant .tau..sub.B of the rotor 6 of the electric motor at the side opposite the load.
The ramping (or linearly accelerated) speed command N.sub.REF is fed to the input, as shown in the block diagram of FIG. 6. When the motor speed N.sub.M and the load speed N.sub.L occur, the transient gain of the speed control system is abnormally raised by the mechanical resonance frequency of the rotational motion, which is determined by the inertia of the individual rotating portions and the torsional rigidity K.sub.2 of the drive shaft 7, if a torsion arises in the drive shaft 7 of the electric motor 2 at the side opposite the load. As a result, periodic speed fluctuations occur which are detrimental to the mechanical facilities and the products during the acceleration of the electric motor, as shown in FIG. 7.
If the load abruptly changes after the end of an acceleration of the electric motor so that a torsion arises in the drive shaft 4 of the electric motor 2 at the load side, the transient gain of the speed control system is abnormally raised by the mechanical resonance frequency of the rotational motion, which is determined by the inertia of the individual rotational portions and the torsional rigidity K.sub.1 of the drive shaft 4. As a result, after the abrupt change of the load speed fluctuations occur which are detrimental to the mechanical facilities and the products, as shown in FIG. 7.
As means for solving this problem, according to the prior art, a vibration suppressing filter 22 is inserted at the output side of the speed controller 11, as shown in FIG. 8, so as to reduce the transient gain at the resonance point of the mechanical system. The torsional vibration suppressing filter 22 is given a transmission function, as expressed by the following formula: EQU 1/{(s/.omega..sub.F0).sup.2 +2.delta..sub.F0 (s/.omega..sub.F0)+1}(2),
wherein: .omega..sub.F0 designates a transient gain reduction starting angular frequency; .delta..sub.F0 designates a filter characteristic constant; and s designates a Laplacian operator.
In the prior art, the filter angular frequency .omega..sub.F0 and the filter characteristic constant, .delta..sub.F0 as appearing in the above Formula, and the proportional gain A of the speed controller 11 are adjusted for all facilities to select a filter constant for reducing the influence of the torsional vibration.
In the prior art, however, the speed control system is made unstable if the filter constants .omega..sub.F0 and .delta..sub.F0 are merely selected for all facilities and adjusted, because the phase delay angle between the speed command and the speed feedback signal of the speed control system is further increased by incorporating the filter. This frequently makes it necessary to drastically reduce the proportional gain A of the speed controller so that the responsiveness of the speed control system is drastically lowered. Thus, there arises the problem that the speed control responding characteristics necessary for the facilities cannot be attained.